This disclosure relates generally to the field of radiation monitoring and dosimetry.
Radiation comes in various forms, including neutrons, x-ray, γ-ray, β-ray or α-particle emission. There are many types of radiation monitors that may be used to determine an amount of radiation exposure, such as ionization detectors, Geiger counters, and thermoluminescent detectors (TLDs). Geiger counters and ionization detectors may determine and display a dose rate (for example, in mRad/hr) or an integrated dose (for example, in Rads) of radiation exposure in real time. Alarm set points may be programmed based on the dose rate or the integrated dose. A Geiger counter or ionization detector may communicate with a computer for data logging or firmware updates. However, Geiger counters and ionization detectors are relatively expensive devices. TLDs allow determination of a dose of radiation based on emission of photons in response to application of heat. TLDs may be comparatively inexpensive, but may only be read after a period of exposure time, typically between one and three months. A degree of radiation exposure may only be determined after-the-fact using a TLD; real time dose information is not available.
A semiconductor, or solid state, radiation monitoring device may comprise a metal-oxide-semiconductor field effect transistor (MOSFET) transistor structure having a gate oxide layer fabricated on bulk silicon. A charge is induced in the FET structure by ionizing radiation exposure and trapped in the gate oxide of the FET by a voltage applied to the gate. The threshold voltage (Vth) of the FET may change according to the amount of trapped holes. A dose of radiation experienced by the solid state radiation monitoring device may be determined by determination of the change in Vth.
A FET radiation detector may be fabricated using a fully depleted silicon-on-insulator (FDSOI) FET device that is capable of detecting doses of various types of ionizing radiation, and that exhibits long-term charge retention that enables long-term tracking of total radiation dosage. The FDSOI radiation detector may be made as small or large as desired using semiconductor wafer fabrication technology, and may have a relatively low power drain. A charge may be induced in a buried oxide (BOX) layer of the FDSOI radiation detector by radiation exposure and trapped by voltage applied to a back gate contact or body contact. Determination of the amount of induced charge through determination of the Vth is then used to determine an amount of radiation exposure experienced by the FDSOI radiation detector. An example of an FDSOI radiation detector is found in U.S. patent application Ser. No. 12/719,962 (Gordon et al.), filed Mar. 9, 2010, assigned to International Business Machines Corporation, which is herein incorporated by reference in its entirety.
In complementary metal-oxide-semiconductor (CMOS) processing, the wafers being processed are subject to various types of ionizing radiation sources during various processing steps, such as reactive ion etching (RIE) or plasma-enhanced deposition of dielectrics (PECVD). Electron-hole pairs are created in the silicon (Si) wafer being processed by the energy released by the ionizing radiation, leaving the holes trapped in silicon dioxide (SiO2) portions of the wafer, or at a Si/SiO2 interface in the wafer, while electrons usually diffuse to ground within a short time (a few to tens of pico seconds). If a significant amount of holes are trapped in the SiO2, the threshold voltage of the device will shift, leading to deviated or degraded device performance. As device size continues decreasing with aggressive CMOS scaling, achieving and maintaining the threshold voltage of relatively small transistors is an important and challenging task. Creation of holes in SiO2 during the CMOS processing compromises the scaling efforts by causing device performance variations.
Additionally, in today's globalized business and marketing network, electronics products may be exposed to ionizing radiation during various activities that occur during normal business activities such as shipping (such as X-ray at airports) or even potentially from sabotage. Such ionizing radiation exposure may be accidental, incidental, or intentional. For example, a high-end microelectronic chip may be designed in a first country, fabricated through a contracting manufacturer at a lab or factory in another country, assembled in a third country, and shipped back to the first country for final assembly into a product such as a vehicle. Throughout the entire manufacturing and supply chain, there exist numerous occasions when the chips may be damaged due to ionizing radiation exposure by sources that cause instant or progressive degradation.